Turning to FIGS. 1 and 2, a conventional system 100 can be seen. This system 100 generally comprises a motor controller 102, an inverter 108, resistor R1, and a motor 112. Motor 112 can have several phases (as shown, is a three-phase motor) and can be a brushless direct current (DC) motor, induction motor, and so forth. The motor controller 102 generally includes a controller 104, space vector pulse width modulator (SVPWM) 106, and analog-to-digital converter (ADC) 110, and inverter 108 generally includes transistor Q1 to Q6, which are controlled by pulse width modulation (PWM) signals PWM1 to PWM6, respectively. In operation, the motor controller 102 provides pulse width modulation through PWM signals PWM1 to PWM6 over consecutive PWM periods TS so as to drive the motor 112 using an control algorithm, and, as shown, this system 100 employs a single shunt (i.e., resistor R1) to measure the current traversing the phases of motor 112, which is used by the control algorithm.
Because a single shunt is employed (as opposed to two or three shunts), the ADC 110 should be fast enough to sample the shunt current, while noise in transient current response should be avoided; this, under most circumstances, is practicable. In FIG. 3A, a voltage vector diagram can be seen, where the bounders of the sectors (i.e., sectors I to VI) correspond to the states of transistors Q1 to Q6 of the inverter (as shown in Table 1 below).
TABLE 1Vector(xyz)Q1Q2Q3Q4Q5Q6V0(000)010101V1(100)100101V2(110)101001V3(010)011001V4(011)011010V5(001)010110V6(101)100110V7(111)101010
As shown, the example voltage vector is located in sector I (having an angle θ). From this voltage vector, there are two resultant projections T1 and T2 that correspond to intervals over which the vectors V1 and V2 are applied over the associated PWM period. These intervals T1 and T2 and vectors V1 and V2 are typically generated by SVPWM 106 based on voltage commands να, and νβ that are issued by controller 104. As shown in FIG. 3B, one-half of each of intervals T1 and T2 (over which vectors V1 and V2 are applied, respectively) located are at each end of the PWM period with the remainder of the PWM period being the zero vector V7 or V0, which yield the set of PWM signals shown in FIG. 3C that drive motor 112.
For high speed applications, this methodology can function quite well, but, for low speed operation of the motor 112, the zero vector region (where there is no shunt current) becomes quite large compared to the PWM period. This means that at least one of the projections of periods (i.e., T1 or T2) can become quite small. For example, as the angle θ for the example voltage vector in FIG. 3A approaches 60°, the interval T1 becomes very small, or, alternatively, as the angle θ for the example voltage vector in FIG. 3A approaches 0°, the interval T2 becomes very small. Because these periods (i.e., as the angle θ for the example voltage vector in FIG. 3A approaches 60°) are so small, ADC 110 may not be able to accurately measure the shunt current, which may be true for any of the shaded error regions of FIG. 4 (which are generally in proximity to vectors V1 to V7).
Thus, there is a need for an improved PWM algorithm.
Some examples of conventional systems are: U.S. Pat. Nos. 5,309,349; 6,735,537; 6,914,409; 7,414,425; 7,898,210; 7,952,310; Huo et al., Improved Single Current Sensing Method and Its Realization Based on ADMCF341 DSP Controller,” Analog Devices Technical Paper, 1995; Kim et al., “Phase Current Reconstruction for AC Motor Drives Using a DC Link Single Current Sensor and Measurement Voltage Vectors,” IEEE Trans. on Power Electronics, Vol. 21, No. 5, September 2006, pp. 1413-1419; Blaabjerg et al.; “Single Current Sensor Technique in the DC Link of Three-Phase PWM-VS Inverters: A Review and a Novel Solution,” IEEE Trans. on Ind. App., Vol. 33, No. 5, September/October 1997, pp. 1241-1253; Jung-Ik Ha, “Voltage Injection Method for Three-Phase Current Reconstruction in PWM Inverters Using a Single Sensor,” IEEE Trans. on Power Electronics, Vol. 24, No. 3, March 2009, pp. 767-775; and Cetin et al., “Scalar PWM implementation methods for three phase three-wire inverters,” Intl. Conf. on Electrical and Electronics Engineering, 2009, pp. 1-447-1-451.